Methods for contactless characterization of semiconductor conductivity by using a single-frequency electric signal are already known.
For example, U.S. Pat. No. 4,286,215 discloses such a method for contactless measurement of conductivity using eddy currents. This type of measurement method of the kind in question is similar to an oscillator using a single-frequency electrical signal. This type of measurement method requires making a reference measurement of the conductivity in darkness and then estimating the imbalance under modulated light. The maximum sensitivity of the conductivity measurement, for doping and for the usual thicknesses of silicon wafers, is around 13 MHz. A system using a single-frequency signal is therefore only suitable for characterization of a single type of material, within a range of thicknesses and of limited conductivity.
The present invention aims to overcome these drawbacks.